Traditional ultrasound imaging method adopts linear scattered fundamental signal to image. However, the fundamental signal is susceptible to phase aberration due to the presence of fat layer in the shallow tissue or skin and will result in low imaging quality. As the sound wave is traveling in human tissues during the imaging process, the wave signal will have finite amplitude distortion or generate harmonic signals when encountering strong nonlinear medium such as microbubble contrast agents. When processing tissue imaging, because the magnitude of harmonic signals is lower than the fundamental signal in the beginning, the scattered harmonic signal will suffer from less phase aberration when penetrating the shallow tissue. Thus, tissue harmonic imaging can provide better contrast resolution because it is less susceptible to phase aberration and thus is broadly used in clinical diagnosis.
The contrast agents being used for harmonic imaging are composed of microbubbles. These small bubbles will have harmonic oscillation to generate lots of strong harmonic signals back to the probe when being excited by sound waves. Clinically, the contrast agents are injected into the blood vessel such that the blood vessel would be filled with micro bubbles to strengthen harmonic signals so as to generate a clearer image of blood vessel structure and blood perfusion. That is, the image contrast is enhanced.
A major difference between ultrasound fundamental signal and harmonic signal is the frequency range of echo signal. If the central frequency of the ultrasound signal travelling into the human body is f0, the imaging method using the frequency signal f0 of the echo signal is called fundamental imaging, but the imaging method using the harmonic signals with higher frequency, such as 2f0, 3f0, is called harmonic imaging. Because these harmonic signals are originated from the nonlinear reaction of the medium to the emitted ultrasound signal, harmonic imaging can be also regarded as nonlinear imaging. As mentioned, it is understood that by using a low frequency filter or high frequency filter to select the frequency range to be received, it is capable to decide whether fundamental imaging or harmonic imaging is performed. The discussion focuses on the analysis of components of second harmonic signal because the second harmonic signal is the strongest one among the various harmonic signals.
Although harmonic imaging is of great importance in clinical diagnosis due to better imaging quality, its weak signal intensity is a major drawback and may significantly degrade imaging sensitivity and penetration. Generally speaking, harmonic signal can be at least 20 db weaker than the fundamental signal even at the focus. Thus, there have been some researches and inventions focusing on using code excitation to enhance harmonic wave intensity. Among the various coding technologies, Golay code is easy to use and is quite applicable to code excitation. Golay code is performed by phase coded sequence. That is, the emitted signal has the phase 0° is represented by the symbol [1], the emitted signal has the phase 90° is represented by the symbol [j], the emitted signal has the phase 180° is represented by the symbol [−1], and the emitted signal has the phase 270° is represented by the symbol [−j]. Golay code featuring phase coding can be easily implemented on the hardware. However, Golay code excitation needs two emitting processes A and B to generate the corresponding echo signals complementary to each other. That is, the autocorrelation results of the two echo signals can be summed to totally remove the sidelobe interference.
Multi-frequency excitation has been developed in ultrasound nonlinear imaging. The feature of multi-frequency excitation is to emit multiple frequency components rather than single frequency component. If only considering the second-order nonlinear components, the ultrasound nonlinear signals generated by multi-frequency excitation will include the second harmonic signals of each emitting frequencies and the inter-modulation signal between the emitting frequencies. Thus, in addition to the second harmonic signals being used in typical harmonic imaging, the inter-modulation signals can also be used for generating image. However, multi-frequency excitation using Golay code excitation for imaging will result in incorrect coding of some harmonic wave components, such as second-order harmonic wave and fourth-order harmonic wave. These incorrectly coded components will interfere with the correctly coded signals to cause degradation in imaging quality.
Take nonlinear imaging using two-bit dual-frequency Golay excitation as an example, it is capable to have the frequency components f2−f1 and 2f1 of the second-order harmonic wave showing the correct code [1, −1] during emission A and the correct code [−1, −1] during emission B as shown in the following table. The above mentioned frequency components of the second-order harmonic wave are usually within the pass band of the probe and thus serve as the major signal components for imaging.
Transmit2nd-order Harmonic4th order HarmonicFrequencyf1f2f2 − f12f1f2 + f1f2 − f12f1f2 + f1Golay code[1, j][1, −j][1, −1][1, −1][1, 1][1, 1][1, −1][1, −1][1, 1][1, 1](Emission A)Golay code[j, j][−j, −j][−1, −1][−1, −1][1, 1][1, 1][−1, −1][−1, −1][1, 1][1, 1](Emission B)
However, as shown in this table, it is understood that among the other harmonic wave components, component f2+f1 of the second-order harmonic wave and components f2−f1 and 2f1 of the fourth-order harmonic wave may not accord with the designed Golay code. The codes are all [1, 1]. These frequency components with incorrect code will result in unremovable sidelobe signals during the compression process and the method nowadays cannot effectively resolve this problem.